With the recent rapid advances in the development of portable/wireless electronic devices, there has been an increasing demand for secondary batteries with high energy density for the purpose of reducing the size and weight of the electronic products.
In response to this demand, lithium ion secondary batteries have been widely used in personal wireless electronic devices, including mobile phones, personal computers, camcorders, portable CD players and personal digital assistants (PDAs), due to their high energy density, high operating voltage, and excellent storage and life characteristics.
Such a lithium ion secondary battery uses a liquid electrolyte composed of LiPF6 and an organic solvent. An aluminum can is used as a packaging material to prevent the liquid electrolyte from leaking from the lithium ion secondary battery and to avoid the danger of explosion of the lithium ion secondary battery associated with the use of the liquid electrolyte.
As a result, an increase in the weight and volume of the lithium ion secondary battery is inevitable. There also still exists a danger of leakage of the liquid electrolyte and explosion.
To overcome such disadvantages, lithium polymer secondary batteries are currently being developed. In a lithium polymer secondary battery, a gel type polymer electrolyte is used or a liquid electrolyte is impregnated into a separator to reduce the danger of leakage of the electrolyte solution. A pouch is employed as an external case to reduce the danger of explosion while enabling weight reduction and slimness.
As widely known in the art, such a pouch structure includes an aluminum laminate pouch as an external packaging material connected to electrode terminals made of metals (for example, Al for cathode and Ni or Cu for anode). The innermost layer of the aluminum laminate pouch is a cast polypropylene (CPP) film produced by polymerization of propylene obtained from naphtha or natural gas cracking.
Before thermal adhesion of the cast film to the metallic electrode terminals, a polar modified polypropylene resin, which is produced by modifying a cast polypropylene resin with a compound having a polar group, is coated on the electrode terminals. The modified polypropylene film is inserted into the aluminum (Al) pouch and is thermally bonded to the electrode terminals made of metals (for example, Al for cathode and Ni or Cu for anode). The modified polypropylene film is also thermally bonded to the cast polypropylene film constituting the innermost layer of the aluminum pouch to seal the battery. This sealing separates the inside of the battery from the outside so that the electrolyte solution can be prevented from leakage. When the thermal bonding is performed, the modified polypropylene resin is melted. Thereafter, the modified polypropylene resin is solidified and shrinks. Portions of the modified polypropylene film laminated in contact with the cast polypropylene film at the lateral sides of the electrode terminals also shrink. This shrinkage leads to unsatisfactory sealing of the battery, failing to prevent leakage of the electrolyte solution.
In other words, a conventional bilayer film structure includes a heat-resistant polypropylene layer in contact with a pouch and a polypropylene layer in contact with the metal terminals. If the heat-resistant polypropylene layer is not crosslinked, sufficient thermal stability of the film structure is not expected. On the contrary, in the case where the heat-resistant polypropylene layer is crosslinked, it is necessary to form an additional polypropylene layer on the crosslinked film. That is, two or more unnecessary processes are further required, entailing high processing cost.
Further, an essential problem of the crosslinked film is relatively poor adhesion to the surface of the pouch upon thermal pressing compared to the uncrosslinked film.
A conventional trilayer film structure includes a polypropylene layer in contact with the pouch, a polypropylene layer in contact with the metal terminals, and a PEN or PET layer as an intermediate layer interposed between the two polypropylene layers. That is, the PP films are formed on both surfaces of the heat-resistant polyester film as an interlayer. The PP films exhibit improved thermal adhesion to the pouch and the metal terminals, but the intermediate polyester film or an adhesive component used for attachment of the layers is susceptible to an electrolyte, causing a serious problem of interlayer peeling.